Method and apparatus for manufacturing flat glass on a bath of molten metal

ABSTRACT

Flat glass is manufactured on a bath of molten material in a partially closed chamber with the glass being cooled as it moves over the molten bath. A gas is circulated within the chamber to maintain a protective atmosphere therein. At least a portion of the circulated gas is withdrawn from one place in the chamber above the molten gas, subjected to a treatment which may be cooling or purifying, and then reintroduced into the chamber at another place above the bath so as to recycle the protective gas.

United States Patent [72] Inventors Gustave Javaux [50] Field of Search65/27, 32, Brussels; 99 A, 168, 182 Gilbert Doqulre, Spy; Marcel Bodart,Nam, n of Bdgmm [56] Relerences Cited [21] Appl. No. 835,584 UNITEDSTATES PATENTS Filed J n 23, 1969 3,337,322 8/1967 Taylor 65/32 Patented1971 3,356,476 12/1967 Gulotta 65/32 x [73] Assignee Glaverbelwatermuhnoimofl, Belgium Prtmary ExammerArthur D. Kellogg [32] PriorityJuly 24, 1968 Att0rneyEdmund M. Jaskiewicz [33] Luxembourg [31] 56578ABSTRACT: Flat glass is manufactured on a bath of molten material in apartially closed chamber with the glass being [54] METHOD AND APPARATUSFOR cooled as it moves over the molten bath. A gas is circulatedMANUFACTURING FLAT GLASS ON A BATH 0F within the chamber to maintain aprotective atmosphere MOLTEN METAL therein. At least a portion of thecirculated gas is withdrawn 22 Claims 7Drawing Figs from one place inthe chamber above the molten gas, subjected to a treatment which may becooling or purifying, and [52] US. Cl then reintroduced into the chamberat another place above 65/99 A, 65/132 the bath so as to recycle theprotective gas. [51] Int. Cl C03b 18/02 Y 46 31 9 4 I 7 I, H

a b GAS SUPPLY 33 i a 39 P77,

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R E Y R U P GAS INVE. "TOR GUSTAV E JAVAUX GILBERT DOQU I RE MARCELBODART METHOD AND APPARATUS FOR MANUFACTURING FLAT GLASS ON A BATH OFMOLTlEN METAL The present invention relates to the manufacture of flatglass on a bath of molten material, more particularly, to a method andapparatus for circulating currents of protective gas above the surfaceof the molten bath.

In the manufacture of flat glass on a bath of molten metal in apartially closed tank or chamber it has been known to circulate aprotective gas through the tank so as to maintain a protectiveatmosphere within the tank above the molten bath. This protectiveatmosphere prevents a number of active elements in the air, such asatmospheric oxygen, from reacting chemically with the molten metal andforming metallic compounds which may form substances detrimental to theglass.

In addition to this protective atmosphere, gases which are inert to themolten metal of the bath have been introduced directly into the bath.Such gases, which may be hydrogen, pass through the molten metal in theform of bubbles and accelerate the extraction in gaseous form of aconsiderable portion of the active elements in the molten metal, such assulphur. These extracted active elements join up with the gaseousbubbles so as to be carried to the surface of the metallic bath. Inorder to prevent contamination of the glass by the subsequent reactionproducts, the inert gases must be introduced at such locations that whenthe bubbles leave the metallic bath they escape directly to the interiorof the tank without encountering the glass.

In spite of these known methods of reducing the formation of substanceswhich might contact the glass, glass still leaves the tank with surfaceareas damaged by contaminating substances.

As a result of extensive study of this problem of glass contamination ithas been suggested that a major reason for the deterioration of theglass may be an excessive concentration of gaseous metallic compoundsformed as a result of the relatively slow flow of those currents ofprotective gases which tend to escape through openings in the walls ofthe tank. These currents may occur in the existing forms of thisapparatus since the quantity of gas introduced into the tank dependslargely on how effectively the tank was sealed. Moreover, because of therelatively random flow of other currents of protective gases within thetank, concentrated groups of gaseous metallic compounds may be entrainedby these random flow currents when they meet the more slowly movingcurrents of protective gases. The concentrated groups of gaseousmetallic compounds may thus be conveyed to that portion of the tank inwhich the bath of metal is at a cooler temperature. As the gases tend toescape from the tank, the gases and the entrained concentrated groups ofmetallic compounds encounter lower temperature walls and coolers so thatportions of the compounds are condensed upon these structures. Theamount of the compounds which are condensed increases in proportion asthe degree of concentration of the metallic compounds increases in theprotective gases. When the metallic compounds are subjected to thereaction of a reducing gas, such as hydrogen, which is generallyassociated with the protective gases, the metallic compounds becometransformed into numerous particles of metal which ultimately drop uponthe surface of the layer of molten glass and thus damage the glass in arelatively large number of places.

It is therefore the principal object of the present invention to providea novel and improved method and apparatus for manufacturing flat glasson a bath of molten metal within a partially closed tank.

It is another object of the present invention to provide a method andapparatus for preventing deterioration or contamination of flat glasswhile it is being manufactured on a bath of molten metal.

According to the present invention there is disclosed a method andapparatus for recycling at least a portion of the protective gascirculating within a tank containing a bath of molten material uponwhich the glass floats. The protective gas is withdrawn from at leastone place above the surface of the molten bath and then at least aportion of the withdrawn gas is introduced into the tank or chamber inat least one other place. The protective gas may comprise a singlegaseous component or a mixture of two or more gaseous components.

By recovering a portion of the protective gas after it has been inprotective relationship with respect to the molten material instead ofpermitting all of the protective gas to escape and be dispersed to theatmosphere and by returning the recovered gas to at least one otherplace in the tank so that the gas can still be used for protecting thematerial, it is now possible to provide a relatively economiccirculation within the tank or chamber of flows of gas which are moreconsiderable and move at greater speeds than those previously known gasflows whose flow within the tank was determined largely by the sealingeffectiveness of the tank. The flow of protective gases as disclosedherein prevents relatively concentrated groups of gaseous metalliccompounds from forming and form moving in a disorderly or random fashionwithin the tank. In addition, by recirculating at least a portion ofthese gas flows at selected locations within the tank the gaseousmetallic compounds forrned at particular places can be conveyed to otherplaces within the tank where they are not able to be transformed intosubstances which are likely to contaminate the glass.

The apparatus according to the present invention may comprise a devicefor treating at least a portion of the gas withdrawn from the tankbefore this withdrawn portion is reintroduced into the tank. Thistreatment permits a higher quality gas to be reintroduced into the tank.The treatment device may comprise a cooling device which thus permitscooling of the surface of the layer of molten glass to be more flexiblycontrolled than with water coolers located within the tank. Thetreatment device may also comprise a purifying apparatus which in turnmay comprise at least two purifying units. One such unit retainssubstances which condense at lower temperatures and the other unitretains substances which do not condense at such lower temperatures. Asa result, the degree of pollution of the protective atmosphere withinthe tank can be kept at a minimum level.

Other objects and advantages of the present invention will be apparentupon reference to the accompanying description when taken in conjunctionwith the following drawings, which are exemplary, wherein;

FIG. 1 is a plan view of a chamber together with gas recycling andpurifying units according to the present inven tion with the tank itselfbeing shown in section on line l-I of FIG. 2;

FIG. 2 is a sectional view of the chamber taken along the line Il-ll ofFIG. 1

FIG. 3 is a sectional view similar to that of FIG. 2 of a modifiedchamber for the manufacture of flat glass according to the presentinvention;

FIG. 4 is a view similar to that of FIG. I with the tank of FIG. 3 beingshown in section on the line lV-IV of FIG. 3;

FIG. 5 is a diagrammatic view of a gas purification apparatus accordingto the present invention;

FIG. 6 is a sectional view taken along the line VIVI of FIG. 7 of an endportion of another form of chamber and shows a portion of a gasrecycling system; and

FIG. 7 is a sectional view taken along the VII-VII of FIG. 6.

Proceeding next to the drawings wherein like reference symbols indicatethe same parts throughout the various views a specific embodiment andmodifications of the present invention will be described in detail.

In FIGS. 1 and 2 the glass melting apparatus is merely represented bythe downstream end of a casting system 2 having a base 3 and upstandingsidewalls 4 and 5 to channel molten glass 6 past a flow regulatingbarrier 7 onto a bath 8 of molten metal in a tank or chamber 9. Tank 9comprises a bottom wall 10, an upstream end wall 11, a downstream endwall 12 and sidewalls l3 and 14. There is also a roof structure 15which, as may be seen in FIG. 2, has a crown 16, an upstream end wall 17and a downstream end wall 18. Positioned between the roof structure andthe flow regulating barrier 7 is a chamber or passage 19 having a topwall and a sidewall 21.

The interior of tank 9 is heated by suitable heating means known in theart and not shown in the drawings to maintain a hot zone 22 at theupstream end of the tank. The temperature within the tank decreases inthe downstream direction. When molten tin is used as the bath 8 thetemperature gradient will be from about l,000 C. near the upstream endwall 11 to about 600 C. near the downstream end wall 12.

Above the surface of the molten bath 8 at the hotter end of the tankthere are two groups of four conduits 24a and 24b which extend throughthe sidewalls 13 and 14 respectively so that their ends 240 and 24d openinto the interior of the tank 9. These conduits are provided with valves25a and 25b and at their outer ends are connected to collectors 26a and26b which are interconnected by a conduit 27.

Extending from the collector 26a is a suction conduit 28 leading to asuction fan 29. In operation, fan 29 continuously withdraws gas from thehotter end of the tank through conduits 24a and 24b and the collectors26a and 26b. The discharge or delivery conduit 30 of fan 29 is connectedto conduits 31 and 32. Conduit 31, which leads to the atmosphere, isprovided with a valve 33 and is the main conduit for evacuating thegases from the tank 9. Conduit 22 is provided with a valve 34 and isconnected to one end of a cooling device 35. Cooling device 35 comprisesan elongated metal chamber 36 having a series of internal baffles 37arranged to provide a tortuous path therethrough for gases. A pipe 38 isalso mounted within chamber 36 and contains a coolant which flows in thedirection indicated by the arrows a and b. The coolant pipe 38 isprovided with control valves 39a and 39b. The other end of coolingdevice 35 is connected through conduit 40, a valve 41 and suctionconduit 42 to a blower 43.

A supply reservoir 46 for protective gas is connected to the suctionconduit 42 by conduit 44 and valve 45. The reservoir 46 is alsoconnected to the inlet end of cooling device 35 by conduits 47 and 49between which is positioned an auxiliary fan 48. Conduit 49 is providedwith a valve 50. The cooling device 35 can be flushed with protectivegas to remove any air therein by operating auxiliary fan 48 while valves34 and 41 are closed and opening valve 50 and a valve 52 in a dischargeconduit 51 near the discharge end of the cooling device.

Blower 43 delivers cooled gas to the cooler or lower temperature end oftank 9 through a conduit 53, distributing boxes and 54b and conduits 56aand 56b. A conduit 55 interconnects the two distributing boxes. Theconduits 56a and 56b are provided with valves 57a and 57b and open intothe tank above the level of the molten bath 8 at openings 56c and 56d.

Auxiliary conduits which are not shown in the drawings are provided forblowing protective gas through the glass entry and exit slots atopposite ends of tank 9 in order to prevent ambient air from enteringthe tank through these openings.

At the exit end of the tank 9, glass sheet 58 is lifted from the moltenbath and conveyed by rollers 59 to an annealing lehr which is not shownin the drawings.

In the operation of the apparatus illustrated in FIGS. 1 and 2, thecooling device 35 is initially flushed with the protective gas whichgenerally consists of a certain proportion of hydrogen. As an example,the protective gas may be a mixture of 95 percent nitrogen and 5 percenthydrogen. This flushing operation removes air from the cooling deviceand the air is replaced by the protective gas.

Prior to melting the metal, which may be tin, to form the molten bath 8,valves 33 and are opened and blowers 29 and 43 are operated to removeair from the tank and to replace the air by the protective gas. When thespace within the tank above the molten bath has been positively filledwith the protective gas the valves 33, and 52 are closed and the valves34 and 40 are opened. As the molten ribbon of glass is being cast uponthe surface of the molten metal bath a strong positive current ofprotective gas flows within the tank from the inlet openings 56c and 56dnear the cooler end of the tank to the outlet openings 24c and 24d nearthe hotter end of the tank. The cooling action of the gas flow on thefloating glass can be controlled to a certain extent by valves 39a and3912 which control the supply of coolant, such as water, through pipe 38and thus influence the extent of cooling of the gases during therecycling operation. The cooling action of the gas flow on the floatingglass will also accelerate the cooling of the surface of the layer ofmolten glass.

It is thus apparent that at least a portion of the protective gas withinthe tank is withdrawn from the tank above the bath at the highertemperature end and reintroduced into the tank above the bath at the lowtemperature end. This current of protective gas is thus in counterflowrelationship to the movement of the ribbon of glass through the tank onthe molten bath.

The valves 33 and 50 can be adjusted to vary the ratio of the mixturebetween the recycled protective gas and fresh protective gas from thesupply reservoir 46.

Proceeding next to FIGS. 3 and 4 the apparatus illustrated therein isprovided with a purifying apparatus 60 instead of the cooling apparatus35 in the embodiment of FIGS. 1 and 2. Only those elements of thismodified apparatus which differ from the structure disclosed withrespect to FIGS. 1 and 2 will be described.

The protective gas enters the tank 9 from four conduits 61, 62a, 62b and62c which extend transversely across the tank and are located atdifferent levels above the bath as shown in FIG. 3. Conduit 61 isprovided with a series of openings 63 which direct the gas leaving thisconduit parallel with the surface of the metal bath and toward thedownstream cooler end of the tank in order to prevent ambient air fromentering the tank through the glass exit slot underneath roof end wall18. The conduits 62a, 62b and 620 are provided with gas dischargeopenings which direct the gas from the respective conduits parallel withthe surface of the metal bath 8 and toward the hotter end of the tank.The apparatus of FIGS. 3 and 4 thus functions similarly to the apparatusof FIGS. 1 and 2 except that the recycled protective gas is free fromimpurities picked up as the gas proceeds through the tank instead ofbeing merely cooled.

It is pointed out that it is not necessary to recycle all of theprotective gas circulating within the tank but only a portion of thisgas may be withdrawn from above the surface of the molten bath in thehot zone of the tank. At least a portion of this withdrawn gas may bepassed through a purifying device and the purified gas then reintroducedinto the cooler zone of the tank. As a result, the quantity ofprotective gas passing through the tank can be increased and the qualityof the currents of gas introduced into the tank can be considerablyimproved. The degree of pollution of the protective atmosphere withrespect to the molten bath can be significantly reduced if at least aportion of the gas taken from above the surface of the molten bath ispassed through at least one device for removing substances which arecondensable at a low temperature and then through at least anotherpurifying device for removing substances which do not condense at lowtemperature, in a manner to be presently described.

In FIG. 5, the purifying apparatus 60 is illustrated in greater detailand includes a conduit 65 at the top of the drawing which may beconsidered to correspond to the conduit 32 leading to the left end ofpurifier 60 in FIG. 4. Conduit 65 leads into a conduit 66 having valves67a and 67b on opposite sides ofitsjunction with conduit 65. Downstreamof valve 67a in the direction of arrow 68 conduit 66 is connected to aconduit 69 through a valve 70 and to a branch conduit 71 which leads toa condenser 72. Condenser 72 comprises a reservoir 73 the upper portionof which is divided into two compartments 74 and 75 by a petition 76 andthe lower portion of which is in the form of a funnel 77 having a bottomvalve 78. Compartments 74 and 75 accommodate cooling devices 79 and 80having coolant flow control valves 81-82 and 83-84 respectively.Electric heating elements 85 are positioned around funnel 77 to maintaina suitable temperature in the lower portion of the reservoir. Condenser72 communicates with a filter 86 through a connecting conduit 86. Afterthe protective gas is cooled in the condenser 72 and filtered it is thenpassed on to an absorber 87 which absorbs hydrogen sulfide from the gas.The gas flows to the absorber through a conduit 88, valve 91a, conduits90 and 93, auxiliary blower 92, conduits 94 and 95, valve 960 andconduit 99.

In order to remove gases which condense at low temperature, for examplevapors of the metal forming the bath or of metal compounds such as SnS,it is only necessary to lower the temperature of the protective gaswithdrawn from the tank.

The elimination of certain polluting gases which do not condense at lowtemperature, such as H 8 and water vapor, can be carried out by bringingthe withdrawn gas into contact with a substance which retains suchpolluting gases.

The absorber 87 includes a metal container 100 charged with a solidsubstance such as caustic soda in flake form or limonite to offer alarge reaction surface to the protective gas. Limonite has the advantageof being more easily handled than caustic soda. The caustic sodaeliminates inter alia sulphides and more particularly hydrogen sulphide.

On leaving the absorber 87, the gas flows to a deoxygenation chamber 101through a valve 103 and a conduit 102. In the deoxygenation chamber 101,the oxygen in the flow of protective gas is reacted with hydrogen in thepresence of a catalyst which promotes a water-forming reaction. Thecatalyst may be alumina impregnated with metallic palladium, palladiummounted on an alumina support in order to increase the reaction surfaceand reduce to a minimum the quantity of metal to be used, or platinum.

Upon leaving the deoxygenating device 101, the gas passes to a coolingdevice 109 through a conduit 110 and a valve 111. The cooling devicereduces the temperature of the protective gas from the value to which ithas been increased by the exothermic reaction in deoxygenation chamber101 to a value below 25 C. prior to floating the gas to a water vaporabsorbing column 116.

The gas flows through a valve 118 in conduit 117 to the column 116 whichcomprises a container 123 having beds of alumina for drying the gas.

Upon leaving the bottom of the water vapor absorbing column 116, the gasflows through conduit 124 and valve 127 into a conduit 129 which is theequivalent of conduit 40 in FIG. 1. The gas flows along conduit 129 tothe cooler end of the tank in which the flat glass is formed.

The apparatus illustrated in FIG. 5 is constructed as one of a pair oftwo similar structures which can be operated ultimately. These twinstructures are linked by the conduits 66, 90 95 and 126 which are shownon the right side of FIG. 5 and are provided with valves 67b, 91b, 96band 128 respectively. When the apparatus shown in FIG. 5 is to beoperated these last-mentioned valves are closed.

Before flowing the protective gas from the tank in which the flat glassis being formed through the purifying apparatus, the several devices ineach of the pair of purifying structures are flushed with freshprotective gas from a reservoir such as 46 in FIG. 1. This accomplishedby closing valves 67a, 78, 91a, 96a, 103, 111, 118 and 127. Theauxiliary blower 48 shown in FIG. 1 thus pumps fresh gas into thepurifying system at the positions marked with the broken line arrows130a which are along conduits 69, 97, 105, 112, and 119 while the valvesin these conduits 70, 98, 107, 114 and 122 respectively are open. Thestreams of flushing gas leave the system at the positions marked withthe broken line arrows 130b which are at valve 89, valve 106 and conduit104, valve 115 and conduit 113, valve 121 and conduit I20, and valve125. In this manner the air in the various devices is replaced by theprotective gas.

After the several devices have been filled with protective gas, thevalves 70, 89, 98, 106, 107, 114, 115, 121, 122, and 125 are closed. Thevalves 67a, 91a, 96a, 103, 111, 118 and 127 are then opened. Auxiliaryblower 92 then operates to draw in protective gas from the tank 9.

The gas from the tank 9 flows in the direction of arrows 68 and entersthe condenser 72. The temperature of the gas before entering thecondenser is kept sufficiently high to prevent premature condensation.As the gas passes through compartment 74 and 75 it is cooled so as tocondense some of the components which may have been extracted frominside the tank, such as tin and tin sulfide when the molten bath ismolten tin. The condensate is collected in the lower portion ofcondenser 72 and maintained in the liquid state by the electric heatingelements 85. When the glow of gas to be purified is switched to theother purifying apparatus of the twin pair, valve 78 is opened toevacuate the funnel portion 77. In the next stage of the purification infilter 86, solid or solidified components such as tin oxide where moltentin is used in the tank are removed.

After being drawn through the condenser 72 and the filter 86 by theauxiliary blower 92 the partially purified gas is pumped by this blowerthrough the absorber 87, the deoxygenation device 101, cooling device109, and the water vapor absorbing column 116 to remove contaminantswhich do not condense at low temperature.

The gas leaving column 116 is thus substantially free from thoseconstituents most likely to form substances which contaminate glass. Thepurified gas is conducted to the cooler end of the tank 9 at atemperature corresponding to ambient temperature. If desired, thetemperature of the purified gas can be raised to a predetermined valuebefore being reintroduced into the tank.

In FIGS. 6 and 7 only a portion of a glass-melting tank furnace 106 isshown at the downstream end of a casting system 132 having a base 133and sidewalls only one of which is shown at 134 in FIG. 6. The castingsystem channels molten glass 135 past a flow-regulating barrier 136 ontoa molten metal bath 137 in a tank 138 only the high temperature end ofwhich is shown. The lower portion of the tank is formed by a bottom 139,an upstream end wall 140 and sidewalls 141 and 142. The upper or roofstructure of the tank is indicated at 143 and comprises a crown 144, anupstream end wall 145 and sidewalls 146 and 147. Within the tank 138there are provided known heating means, not shown in the drawings, tomaintain the desired temperature with a temperature gradient along themolten metal bath which can be regulated.

Above the surface of the molten metal bath 137 in the portion of thetank comprising the hot zone 148 are two groups of four gas withdrawalconduits 149a and 14% which extend through the sidewalls 141 and 142respectively so as to open into the tank at 149C and 149d. The conduitsare provided with valves 150a and l50b and their outer ends areconnected to collectors 151a and 151b respectively which areinterconnected by a conduit 152. A suction conduit 153 leads fromcollector 151b to a fan 154 which delivers the gas withdrawn from thetank along conduit 155. At least a portion of such gas is passed througha cooling or purifying apparatus which is not shown in FIGS. 6 and 7 butwhich may be similar with the cooling or purifying structures describedabove.

At least a portion of the gas withdrawn from the tank above the hot zonethrough conduits 149a and 1491) is subsequently reintroduced into thetank at other positions in its hotter end. Thus, at least a portion ofthe gas is reintroduced into the tank 138 by two groups of threeconduits 156 and 157 which extend through the crown 144 and open intothe top of the tank. One of such groups of pipes is shown at 156a, 156band l56c in FIG. 7. Along these conduits is a second group of threeconduits one of which 157b is shown in FIG. 6. The gas arrives at theseinlet conduits through conduit 158 which is connected to the outlet ofthe cooling or purifying apparatus, pump 159, conduit 160, collector161, conduits 162 (only one of which is shown), distributing boxes 163and 164 and valves 165 and 166 which are indicated with the appropriateone of these numerals followed by the corresponding letter notation tocorrespond to the respective inlet conduits.

In the operation of the apparatus as shown in FIGS. 6 and 7, strongpositive currents of protective gas are maintained in the hotter end ofthe tank flowing downwardly from the top inlets to the side outlets. Asa result, impurities in the tank at mosphere at the hotter end thereofare rapidly withdrawn so that a very high quality atmosphere ismaintained in this part of the tank. To prevent movement ofgas-containing impurities along the tank toward its cooler end, some ofthe protective gas withdrawn through conduits 149a and 1149b may, aftercooling or purification, be reintroduced into the tank at its cooler endso that a flow ofv gas at a relatively low velocity is also maintainedwithin the tank from the cooler to the hotter end thereof. By way ofexample, if eight units of protective gas are withdrawn per unit timethrough the side outlets at the hotter end of the tank, six units ofthis gas may be reintroduced per unit time into the tank at the top ofits hotter end. The remaining two units may be reintroduced into thetank at its cooler end.

For the various forms of apparatus disclosed herein it is preferable tocontrol the amount of gas withdrawn for recycling with respect to theimpurity content of the protective gas at a predetermined point withinthe tank. The quality of the atmosphere can thus be flexibly andeconomically controlled while keeping the degree of pollution withinpredetermined limits. Apparatus for thus regulating the amount of gaswithdrawn for recycling may thus be provided.

It will be understood that this invention is subject to modification inorder to adapt it to different uses and conditions.

What is claimed is:

I. In a method of manufacturing flat glass on a bath of molten materialin a partially closed chamber where the glass is cooled as it moves overthe molten material, the steps of circulating a gas within the chamberto maintain a protective atmosphere therein, withdrawing at least aportion of the circulated gas from above the surface of the ,hotter zoneof the bath, purifying at least a portion of the withdrawn gas byremoving from the withdrawn gas pollutant material which does notcondense at low temperatures by bringing the withdrawn gas into contactwith a cleaning agent, the cleaning agent being selected from the groupconsisting of caustic soda and limonite, and introducing the purifiedgas back into the chamber.

2. In a method as claimed in claim 1 wherein the gas is withdrawn fromabove the surface of the hotter zone of the bath, cooling at least aportion of the withdrawn gas, and introducing the cooled gas back intothe chamber at a cooler zone therein.

3. In a method as claimed in claim 1 wherein the gas is withdrawn fromabove the surface of the hotter zone of the bath, purifying at least aportion of the withdrawn gas, and introducing the purified gas back intothe chamber at a cooler zone therein.

4. In a method as claimed in claim 3 with the step of removing gaseouspollutant material which may be condensable at a low temperature from atleast a portion of the withdrawn gas, and removing from the withdrawngas pollutant material which does not condense a low temperatures.

5. In a method as claimed in claim 4 with the step of lowering thetemperature of the withdrawn gas to remove gaseous pollutant materialwhich condenses at a low temperature.

6. In a method as claimed in claim I wherein the caustic soda is inflake form.

7. In a method as claimed in claim 3 with the step of subjecting thewithdrawn gas to a deoxygenation treatment.

8. In a method as claimed in claim 7 wherein the withdrawn gas isbrought into contact with a catalyst promoting a waterforming reaction.

9. In a method as claimed in claim 8 wherein the catalyst is palladium.

10. In a method as claimed in claim 8 wherein the catalyst is on analumina support.

11. In a method as claimed in claim 1 and the step of regulating thequantity of gas being recycled with respect to the impurity content ofthe gas at a predetermined point in the chamber.

12. In a method as claimed in claim 1 wherein the gas is withdrawn fromabove the surface of the hotter zone of the bath, cooling at least aportion of the withdrawn gas before bringing the withdrawn gas intocontact with the cleaning agent, and introducing the cooled gas backinto the chamber at a cooler zone therein.

13. In an apparatus for the manufacture of flat gas on a bath of moltenmaterial in a partially closed chamber having a protective atmosphere ofgas therein with the glass cooling as it moves on the bath, thecombination of means for withdrawing gas from the chamber from above thebath of molten material, means for introducing withdrawn gas into saidchamber at another place so that the protective gas is recirculatedwithin the chamber, and means for treating at least a portion of thewithdrawn gas before reintroducing the gas back into the chamber, saidtreating means comprising a purifying device comprising means forretaining pollutant material which does not condense at a lowtemperature, said purifying means further comprising a tank havingtherein a cleaning agent which retains a least a portion of saidpollutant material, said cleaning agent being selected from the groupconsisting of caustic soda and limonite.

14. In an apparatus as claimed in claim 13 wherein said treating meanscomprises a cooling device.

15. In an apparatus as claimed in claim 13 wherein said purifying devicecomprises a first means for retaining pollutant material which condensesat a low temperature and a second means for retaining pollutant materialwhich does not condense at a low temperature.

16. In an apparatus as claimed in claim 15 wherein said first purifyingmeans comprises a cooler, and means for circulating a coolant throughsaid cooler.

17. In an apparatus as claimed in claim 13 wherein said caustic soda isin flake form.

18. In an apparatus as claimed in claim 13 wherein said treating meanscomprises means for removing oxygen from the withdrawn gas.

19. In an apparatus as claimed in claim 18 wherein said oxygen removingmeans comprises a tank having therein a catalyst for promoting awater-forming reaction.

20. In an apparatus as claimed in claim 19 wherein said catalyst ispalladium.

21. In an apparatus as claimed in claim 19 and an alumina support uponwhich said catalyst is deposed.

22. In an apparatus as claimed in claim 13 and means responsive to theimpurity content of the protective gas at a' predetermined point in thechamber for regulating the quantity of gas withdrawn from the chamberfor recirculation therethrough.

2. In a method as claimed in claim 1 wherein the gas is withdrawn fromabove the surface of the hotter zone of the bath, cooling at least aportion of the withdrawn gas, and introducing the cooled gas back intothe chamber at a cooler zone therein.
 3. In a method as claimed in claim1 wherein the gas is withdrawn from above the surface of the hotter zoneof the bath, purifying at least a portion of the withdrawn gas, andintroducing the purified gas back into the chamber at a cooler zonetherein.
 4. In a method as claimed in claim 3 with the step of removinggaseous pollutant material which may be condensable at a low temperaturefrom at least a portion of the withdrawn gas, and removing from thewithdrawn gas pollutant material which does not condense at lowtemperatures.
 5. In a method as claimed in claim 4 with the step oflowering the temperature of the withdrawn gas to remove gaseouspollutant material which condenses at a low temperature.
 6. In a methodas claimed in claim 1 wherein the caustic soda is in flake form.
 7. In amethod as claimed in claim 3 with the step of subjecting the withdrawngas to a deoxygenation treatment.
 8. In a method as claimed in claim 7wherein the withdrawn gas is brought into contact with a catalystpromoting a water-forming reaction.
 9. In a method as claimed in claim 8wherein the catalyst is palladium.
 10. In a method as claimed in claim 8wherein the catalyst is on an alumina support.
 11. In a method asclaimed in claim 1 and the step of regulating the quantity of gas beingrecycled with respect to the impurity content of the gas at apredetermined point in the chamber.
 12. In a method as claimed in claim1 wherein the gas is withdrawn from above the surface of the hotter zoneof the bath, cooling at least a portion of the withdrawn gas beforebringing the withdrawn gas into contact with the cleaning agent, andintroducing the cooled gas back into the chamber at a cooler zonetherein.
 13. In an apparatus for the manufacture of flat gas on a bathof molten material in a partially closed chamber having a protectiveatmosphere of gas therein with the glass cooling as it moves on thebath, the combination of means for withdrawing gas from the chamber fromabove the bath of molten material, means for introducing withdrawn gasinto said chamber at another place so that the protective gas isrecirculated within the chamber, and means for treating at least aportion of the withdrawn gas before reintroducing the gas back into thechamber, said treating means comprising a purifying device comprisingmeans for retaining pollutant material which does not condense at a lowtemperature, said purifying means further comprising a tank havingtherein a cleaning agent which retains a least a portion of saidpollutant material, said cleaning agent being selected from the groupconsisting of caustic soda and limonite.
 14. In an apparatus as claimedin claim 13 Wherein said treating means comprises a cooling device. 15.In an apparatus as claimed in claim 13 wherein said purifying devicecomprises a first means for retaining pollutant material which condensesat a low temperature and a second means for retaining pollutant materialwhich does not condense at a low temperature.
 16. In an apparatus asclaimed in claim 15 wherein said first purifying means comprises acooler, and means for circulating a coolant through said cooler.
 17. Inan apparatus as claimed in claim 13 wherein said caustic soda is inflake form.
 18. In an apparatus as claimed in claim 13 wherein saidtreating means comprises means for removing oxygen from the withdrawngas.
 19. In an apparatus as claimed in claim 18 wherein said oxygenremoving means comprises a tank having therein a catalyst for promotinga water-forming reaction.
 20. In an apparatus as claimed in claim 19wherein said catalyst is palladium.
 21. In an apparatus as claimed inclaim 19 and an alumina support upon which said catalyst is deposed. 22.In an apparatus as claimed in claim 13 and means responsive to theimpurity content of the protective gas at a predetermined point in thechamber for regulating the quantity of gas withdrawn from the chamberfor recirculation therethrough.